Physical vapor deposition (PVD or sputtering) is a well-known process for depositing thin solid films on substrates, and is widely practiced in the semiconductor industry. Ionized physical vapor deposition (IPVD), also referred to as ionized metal plasma (IMP) deposition, has been used more recently to deposit metal films (notably copper) in etched structures such as vias and contact holes. IMP deposition apparatus differs from earlier versions of PVD apparatus in that a dense plasma is used to ionize the metal atoms during the sputter deposition process.
A typical IPVD arrangement is shown schematically in FIG. 1. In an IPVD apparatus 100, a flux of metal atoms 101 is produced by sputtering from a target 102. These atoms are ionized by passing through a plasma 103, and are deposited on a sample 1 (typically a silicon wafer). The ions 104 are transported from the plasma to the wafer by means of a negative potential on the sample relative to the plasma. IPVD devices differ from conventional PVD devices in that during an IPVD process, the metal ions have additional kinetic energy because of the potential difference between the plasma and the sample (typically 5 to 15 volts).
In order to maintain a uniform potential difference between the plasma and the wafer, and to control the temperature of the wafer, the wafer is clamped against a cooled pedestal 111 using a clamp ring 110. In one arrangement known in the art, the pedestal is mounted on an elevator mechanism, and is moved vertically upwards from an "idle" position to a "process" position where the weight of the clamp ring is borne by the outer edge of the wafer. (FIG. 1 shows the wafer and pedestal in the "process" position.) The clamp ring is typically several pounds in weight and has an inside diameter slightly smaller than the diameter of the wafer.
The plasma potential is always more positive than the potential of the sample or the various fixtures in the process chamber, including the clamp ring 110. The clamp ring is in mechanical and electrical contact with the wafer 1; a flexible conductor 114 touching the pedestal and the underside of the clamp ring ensures electrical contact therebetween. Accordingly, the clamp ring is subject to ion bombardment from the plasma, which tends to heat the clamp ring. In addition, the sample 1 and pedestal 111 are often connected to a biasing power source (not shown); a portion of the bias power is dissipated in the clamp ring 110, further adding to heating of the clamp ring. For a typical commercial 200 mm wafer IPVD system, the clamp ring might receive as much as 50% of the applied bias power of 500 W. Measurements using thermocouples attached to the clamp ring have shown that the clamp ring may reach a temperature of 300.degree. C. during a deposition process.
Heating of the clamp ring is undesirable for several reasons. Thermal expansion of the ring, relative to the wafer, results in changes in the size of the shadowed edge region of the wafer. The clamp ring may warp due to thermal expansion, thus shortening its operating life.
Furthermore, since the clamp ring is subject to metal ion bombardment, a metal film is deposited on the clamp ring which typically has thermal expansion properties different from those of the clamp ring material. The heating of the clamp ring is cyclical (heating during a deposition process, cooling between deposition processes). This results in stresses in the films deposited on the clamp ring, which in turn may result in the film breaking and forming particles.
Active cooling of the clamp ring is difficult, since the IPVD process chamber is a clean, ultrahigh vacuum environment. The ring is moved within the process chamber to clamp and unclamp the wafer at the beginning and end of each deposition process. Accordingly, using water-cooling lines in contact with the clamp ring is not desirable, since those lines would flex with the movement of the clamp ring, and eventually leak or generate particles.
Another problem related to the clamp ring is that of clamp ring/wafer sticking. This effect has been observed in IPVD of copper and aluminum films. As shown in FIG. 1, a typical clamp ring has a portion 110a which overhangs the point of contact between the wafer and the clamp ring, thereby protecting this point from film deposition so that the wafer does not "weld" to the clamp ring. However, if the clamp ring does not properly contact the wafer (due to thermal expansion, eccentric wafer location on the pedestal, etc.) one side of the wafer may instead contact surface 110b on the overhanging portion of the clamp ring. In that event the wafer may stick to surface 110b. This usually leads to breakage of the wafer. Besides the loss of a valuable product wafer, the IPVD tool becomes contaminated with wafer debris and is unavailable for a substantial period of time.
There is a need for an improved wafer clamp ring arrangement in IPVD devices which reduces or avoids the problems of clamp ring heating and clamp ring/wafer sticking.